<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=  , initial-scale=1.0">
    <title>Document</title>
    <script src="js/vue.js"></script>
</head>
<style>
    .active{text-decoration: underline;}
    .text-danger{font-weight: bold;}
</style>
<body>
    <div id="app">
        <div v-bind:class="{active:isActive,'text-danger':hasError}">Hello</div>
        <div v-bind:class="classObj">Vue.js</div>
    </div>

</body> 
<script>
        var vm = new Vue({
            el:'#app',
            data:{
                 isActive:true,
                 hasError:true,
                 classObj:{
                    isActive:true,
                    'text-danger':true,
                 }
            }
        })
    </script>
</html>